Many different types of body-implantable, automatic cardiac pulse generator systems, sometimes referred to as "pacemakers,` are known and commercially-available. In general, cardiac pulse generators are devices used to supplant some or all of a malfunctioning heart's natural pacing function. Cardiac pulse generators are typically coupled to a patient's heart via one or more conductive leads. for communicating cardiac stimulating pulses from the pulse generator to the heart, and for conveying sensed cardiac electrical signals to sensing circuitry associated with the pulse generator.
Over the years, the functional capabilities and complexities of cardiac pulse generator systems have increased dramatically. Early body implantable cardiac stimulating devices were fixed-rate, non-inhibited pulse generators which operated to deliver electrical stimulating pulses to the patient's heart at regular intervals, without sensing of, and hence without regard to, intrinsic cardiac electrical activity.
Later, so-called "demand" pacemakers were developed. Demand pacemakers include sensing circuitry for monitoring intrinsic cardiac activity, so that the pulse generator can be inhibited on a beat-to-beat basis, i.e., prevented from delivering stimulating pulses when the heart is functioning properly.
A further development in pulse generator functionality involved variable rate stimulation. Variable rate pulse generators can include means for sensing certain physiologic conditions of the patient e,g, blood oxygenation levels, activity levels and the like, such that the rate at which pacing pulses are delivered can be dynamically varied in accordance with the patient's physiologic demand.
Advances in electronics and semicondonductor technology over the years have been such that it has become increasingly possible and practical to make "multiprogrammable" implantable pulse generators i.e. pulse generators capable of being programmed to operate in any one of a plurality of different operational modes, and to operate with numerous optional diagnostic and therapeutic features. Such features are typically capable of being activated or deactivated as desired, as will be hereinafter discussed in further detail. Multiprogrammable pulse generators are capable of selectively operating in accordance with any of a plurality of pacing algorithms or pacing "modes."
The Elite II.TM. Models 7084, 7085, and 7086, commercially-available from Medtronic, Inc., Minneapolis, Minn. (assignee of the present invention) are good examples of state-of-the-art multiprogrammable implantable pacemakers. The Elite II.TM. pacemakers are operable in any one of the following modes (where, in each case, the first letter identifies either a single-chamber (S), dual-chamber (D) or neither-chamber (O) mode, the second letter corresponds to the chamber(s) sensed, the third letter identifies the pacemaker's response to a sensed event--either triggered (T), inhibited (I) or dual (D), and the fourth letter, if any, indicates a rate responsive feature): DDR, DDD, DDIR, DDI, DVIR, DVI, SSIR, SSI, DOOR, DOO, SOOR, and ODO.
The Elite II.TM. pacemakers have numerous physician-programmable parameters, as set forth in the following Table 1 and maybe considered typical of some cardiac pacemaker programmable parameters. Current version's of pacemakers generally have more such parameters but Table 1 contains a good heuristic set.
TABLE 1 ______________________________________ PROGRAMMABLE RANGE OF PARAMETER PROGRAMMABILITY ______________________________________ Activity Threshold Low to High, in five settings Rate Responsive 1 to 10 Acceleration Time 0.25, 0.5, or 1.0 minutes Declaration Time 2.5, 5, or 10 minutes Lower Rate in rate responsive modes 40-90 PPM Lower Rate in other modes 30-130 PPM Upper Tracking Rate (atrial tracking) 80-180 PPM Upper Activity Rate (sensor tracking) 80-180 PPM Temporary Rate 30-400 PPM Pulse Width 0.06-1.5 mSec Pulse Amplitude 0.8-5.0 V Atrial Sensitivity 0.54.0 mV Ventricular Sensitivity 1.5-9.0 mV A-V Delay after A-pace (PAV) 30-350 mSec A-V Delay after A-sense (SAV) 30-350 mSec Rate Adaptive A-V Delay On or Off Post-Ventricular Atrial Refractory 160-500 mSec Period (PVARP) Atrial Refractory Period (ARP) 160-470 mSec Atrial Blanking Period 20-40 mSec Ventricular Safety Blanking 20-40 mSec Pacing Polarity Unipolar or Bipolar Sensing Polarity Unipolar of Bipolar Temporary Inhibit On or Off ______________________________________
For a given operational mode, and given a desired set of programmed parameter values, there may in addition be selectively activatable features that are provided to enhance the therapeutic benefit of the pacemaker system. Such features, when activated, may function, for example, to dynamically (e.g. on a cycle-to-cycle basis) adjust a programmed parameter value under certain predefined circumstances or in response to the occurrence of certain predefined combinations of events. One well-known example of such a feature is a Rate Response feature, which operates to adjust the base pacing rate parameter for a pacemaker in accordance with a Rate Responsive function applied to the output of an activity sensor.
Ranges and rates of the various rates and parameters will of course be specific to each pacemaker product, as will the kind and number of variable parameters used in each such product. For example, there is no parameter related to what information to output through and responsive to telemetric communication for example, nor is there any parameter having to do with the type of rate smoothing to apply, whether to sense for vasovagal syncopy and so on. The parameter types which could conceivably be employed and adjusted through the mechanism for adjusting table 1's parameters will be apparent to one of ordinary skill in this art.
Programmable operational parameters such as those listed in Table 1 are used by a pacemaker's control circuitry (e.g. , a custom microprocessor or the like) in order to cause the pacemaker to operate in accordance with pacing algorithm. That is, the mode selected (e.g., DDD, DDI, etc . . . ), along with the programmed parameters (e.g., SAV, PAV, PVARP, etc. [These acronyms are defined in the table and previous pages]) and selected features, described as therapy features below define a pacing algorithm or machine state which determines the pacemaker's operational behavior.
The Elite II.TM. pacemakers, and more state-of-the-art implantable pulse generators, are provided with a telemetry system for facilitating non-invasive programming of the implanted device's operational modes, programmable parameters, and for control of various selectable diagnostic functions, such as those noted above. An external programming unit, such as the Model 9790 Programmer commercially-available from Medtronic, Inc., communicates with the implanted device via radio-frequency signals. Implantable device telemetry systems for facilitating bi-directional communication between an implanted device and an external programming unit are well-known in the art. The telemetry system enables a clinician, using an external programming unit, to program desired values for the various programmable operational parameters, to activate and deactivate the various optional pacing therapy features supported by the implant, and to perform various diagnostic procedures.
Those of ordinary skill in the art will appreciate that for state-of-the-art multiprogrammable pacemaker systems capable of performing numerous different complex operations, often decisions and trade-offs must be made in connection with the selection or activation of various features, modes of operation, and programmed parameter values. In the prior art, many such decisions and trade-offs are commonly made at the programmer-interface level, i.e., during programming of the device at the pacemaker clinic by the doctor operating a programmer. From the programming clinician's standpoint, this can increase the complexity and difficulty of programming an implanted device appropriately for a given patient.
One source of this complexity stems from the existence of certain programmable modes and features of multiprogrammable pacemaker systems that are mutually incompatible with others, For example, if two features that function to dynamically adjust the same operating parameter of a pacemaker based on different criteria are activated at the same time, operation of one feature could potentially interfere with the operation of the other, and vice versa. One feature may, under certain circumstances be attempting to adjust an operating parameter upwards, while the other one, under the same circumstances, is attempting to adjust that parameter downwards. In that case the intended benefits of both features would potentially not be realized.
As a result of the potential for such conflicts, one solution shown in the prior art has been to make it impossible for the programming clinician to activate mutually conflicting features or to program the implant into mutually-exclusive modes. This has been accomplished in many cases through the use of "programmer interlocks," i.e., safeguards written into the software controlling operation of the programming unit itself that prevent the programmer from issuing impermissible combinations of programming commands to a given implant. Thus, for example, if two selectively activatable features of a pacemaker are deemed by the designer or manufacturer of the pacemaker system to be incompatible, safeguards in the programming unit's software would render the programming unit incapable of activating both of these features in the same implant.
One potential disadvantage of the "programmer interlock" approach to avoiding the selection of incompatible modes or features of an implant is that it limits the clinician's discretion to program an implanted device as he or she deems appropriate for a particular patient. Sometimes incompatible features may only need to run occasionally, so the interlock can prevent a clinician from programming in a potentially therapeutic combination. Another potential disadvantage of this approach is that it can increase the complexity and difficulty of programming for the clinician, and leave a potential source of clinician error in place.